renting car at Florence airport and traveling to Siena and other towns for day

There's nothing wrong with having a dream. You just have to hope for decent weather if you plan to restrict yourself to one day. For places to visit, my list would be too long to post here. Can't imagine how such a list would be any better than a Google list. I recommend viewing Google Images and see what moves you. If you share your likes and dislikes, strangers will get a better sense of what to recommend.

There are many options, although in just a day you may not have the time to visit many. Siena alone has quite a bit to offer to the visitor and will take a good portion of the day.
One option I would consider is to get to Siena as soon as possible in the morning, as the Museo Civico inside the Palazzo pubblico, probably the most important, opens from 10 to 6pm. If you get there earlier (but I doubt it, as rental car offices open at around 8 and the drive is one hour) you can start roaming around Piazza del Campo or the Duomo.
Once Siena is covered you could return to Florence Via the Regional Road SR222 aka Via Chiantigiana (Chianti Road). If you are interest in wineries there are many there, as well as nice picturesque towns.
If wine or vineyards dont interest you, then you could return via Monteriggioni and San Gimignano for two of the most famous medieval towns in Tuscany.

For renting a car, I would avoid the airport. It costs more to rent there, it’s farther, and the opposite side of town if you want to go to Siena. I would rent from downtown at the Borgo Ognissanti location.
Or, if you rent from Hertz, they have a new office on via Ambrosoli, which is next to the Varlungo Bridge in the Rovezzano district, the easternmost bridge in Florence. From there you can hop on Viale Marco Polo in a sec and head south.

The airport rental center hours are 8:30-23:30.
The downtown location at Borgo Ognissanti hours are 8:00-19:00 (after hours drop off possible with prior arrangements)

You cannot park or drive in most city/town centers. Therefore if you actually want to visit towns, not just see them as you drive past, pick one or two. You will have to park and walk in order to actually visit.